1. Field of the Invention
The present invention relates to semiconductor devices, light-emitting devices, and display devices. The present invention relates to methods for driving such devices and methods for manufacturing such devices. Examples of semiconductor devices are semiconductor devices including active elements such as transistors. Examples of light-emitting devices are light-emitting devices including light-emitting elements such as electroluminescence elements (hereinafter referred to as EL elements). Examples of display devices are display devices including display elements or light-emitting elements such as EL elements. In particular, the present invention relates to semiconductor devices, light-emitting devices, and display devices that are less influenced by variations in characteristics of transistors.
2. Description of the Related Art
Since light-emitting devices including light-emitting elements have high visibility, are suitable for reduction in thickness, and do not have limitations on viewing angles, the light-emitting devices have attracted attention as display devices that are alternatives to cathode ray tubes (CRTs) or liquid crystal display devices. Specifically proposed structures of active matrix display devices including light-emitting elements differ depending on manufacturers. In general, at least a light-emitting element, a transistor (a switching transistor) that controls input of video signals to pixels, and a transistor (a driving transistor) that controls the amount of current supplied to the light-emitting element are provided in each pixel.
When all the transistors in the pixels have the same polarity, it is possible to omit some of steps of forming the transistors, e.g., a step of adding an impurity element imparting conductivity to a semiconductor layer. Patent Document 1 discloses a display device in which transistors included in pixels are all n-channel transistors.